High-density concrete is used extensively in nuclear power plants for radiation shielding against biological hazards. Apart from the basic physical properties (i.e., compressive strength, density, and absorption), knowledge of thermal properties of such concrete is required to access its performance under service conditions. This paper describes investigative work carried out on a high-density concrete. Also, because aggregates are the most important and critical component of the high-density concrete, properties of the aggregates and the criteria for their use in concrete have been discussed. In addition, cores were obtained from a shielding wall of a 20-year-old nuclear plant and selective tests were carried out on the core samples for comparison. The mix proportions of the concrete from the wall were different from those of the laboratory-made concrete. Thermal properties, such as conductivity, diffusivity, specific heat, emissivity and coefficient of thermal expansion, were determined, and the values were compared with a normal-density dolomitic-limestone based concrete.